1. Field of the Invention
The invention relates to an internal broach for internally broaching profiles defined by a bottom and flanks, in particular female serrations, in a work piece, comprising a shank, which leads in a direction of broaching and has a central longitudinal axis; and a toothed section with several rows of broach cutting teeth, the rows being disposed successively counter to the direction of broaching; with successive broach cutting teeth being allocated to each other for broaching a profile of a depth; with the broach cutting teeth having bottom cutting blades and first and second sides; with the bottom cutting blades of successive and associated broach cutting teeth having a pitch a relative to the broach cutting teeth that lead in the direction of broaching; with a bottom-cutting-blade relief surface being allocated to the bottom cutting blades; and with the first and second sides passing through the bottom-cutting-blade relief surfaces while forming first and second edges.
2. Background Art
The internal broaches conventionally used for internal profile broaching are known from DIN 1415 (ed. 1973), sheet 1, page 2. They have a shank, a toothed section and a tail end. The shank is held by a broaching machine puller, which pulls the broach through a work piece held in the broaching machine, broaching the profiles in doing so. After the broaching operation, the tail end is seized by a retriever of the broaching machine, which returns the broach after the broaching operation into its initial position. Counter to the direction of broaching, the toothed section comprises several rows of broach cutting teeth, as a rule a great number of these rows of broach cutting teeth. The broach cutting teeth have blades for cutting the bottom of a profile and blades for cutting the flanks of the profile. The broach cutting teeth are disposed successively counter to the direction of broaching and are allocated to each other in this regard, serving to machine a profile; they are progressively stepped in depth i.e., they have a diametric pitch, so that all the broach cutting teeth that serve for machining a profile will successively cut a chip for producing the bottom of a profile.
Since the main machining operation is delivered by the bottom cutting blades, they are also called primary blades. For machining the flanks of a profile, the broach cutting teeth, which are disposed successively counter to the direction of broaching, have flank cutting blades of a back taper as illustrated in DIN 1415 (ed. 1973), sheet 1, page 3, picture 11. The flank cutting blades are also called secondary blades. The back taper is produced by the flank cutting blades of a subsequent tooth being relieved laterally as compared to the flank cutting blades of a leading tooth, so that the flank cutting blades of a subsequent tooth only machine the area provided by diametric pitch or back taper, and do not engage with the work piece in the area where the flank cutting blades of the leading tooth have machined. This helps prevent the broach cutting teeth from being clamped in the vicinity of the profile flanks during the broaching operation. The result is a stepped surface structure of the profile flanks.
The profiles produced by a familiar and customary internal broach have sufficient surface quality, accuracy of profile shape and flank curve for standard applications and requirements. During the broaching operation, displacement of the axis of the broach may occur so that each of the successively engaging teeth has a varying center position relative to the work piece that is to be machined. In particular in the case of twist broaching (helical broaching), torsional deviation may be superimposed on such a displacement of the axis of the broach; the torsional deviation is caused by rotatory forces during twist broaching. Very often, profile accuracy and flank surface quality are not sufficient in this case, flank curve accuracy being satisfactory as a rule. High accuracy of profile shape and flank curve are demanded in particular in the case of running gears such as female serrated gears with spur teeth or helical teeth.
In order to remedy the mentioned deficiencies in the case of correspondingly high demands, it has been familiar practice to provide the broach with a sizing section downstream of the back-tapered broach cutting teeth—seen in the direction of broaching. Such a sizing section comprises several successive broach cutting teeth of identical height, which do not regroove the bottom of the profile. However, they have tooth thicknesses that increase counter to the direction of broaching i.e., all the sizing teeth cut a chip over the full height of the profile flank, the chip thickness generally being 10 to 20 μm. Each flank cutting blade of the sizing teeth must have a relief produced by grinding i.e., it must have a relief angle. They are relief-ground. Sizing helps obtain excellent accuracy of profile shape and high surface quality. Flank curve accuracy deteriorates as compared to the profile broached by progressive stepping. This is due to the fact that the relief-ground flank cutting blades of the sizing teeth are sharp cutting edges of comparatively bad self-guidance behavior.
It is inherent in the system that any changeover from a progressively stepped broaching operation to full form sizing is accompanied with a break in the broaching force, which leads to considerable drawbacks, in particular in the case of twist broaching. Relieving the main cutting force that acts counter to the direction of broaching will lead to a reduction in torsional stress i.e., the torsion of the work piece relative to the internal broach changes. This change may be sufficiently strong so that the full form sizing section is not led correctly into the progressively broached profiles and, as a result, machines the flanks unilaterally so that the profile is not sized on both flanks. Owing to the mentioned deficiencies of the profile produced by progressively stepped broaching, the flank cutting blades of the broach cutting teeth will cut irregularly into the stepped flanks of the profile, with torsional vibrations originating which may negatively affect flank curve accuracy.
U.S. Pat. No. 2,986,801 teaches an internal broach for internally broaching female serrations, which are defined by a bottom and flanks, in a work piece. The bottom cutting blades of successive and associated broach cutting teeth have a positive diametrical pitch as compared to the broach cutting teeth that lead in the direction of broaching. The flank cutting blades of successive and associated broach cutting teeth have a negative diametrical pitch over their full height. This means that the contours of successive broach cutting teeth narrow as the diameter grows. The resulting drawbacks correspond to the drawbacks specified above in connection with DIN 1415 (ed. 1973).
For elimination of the above-mentioned drawbacks in an internal broach, U.S. Pat. No. 5,865,569 teaches to obtain improved surface quality and accuracy of profile shape and flank curves in that the flank cutting blades of successive and associated broach cutting teeth, over their full height, have a pitch that is small relative to the diametrical pitch of the bottom cutting blades. In this known embodiment, the chip space bottom of the broach cutting teeth must be below the web of the profile i.e., below the top circle of the profile that is to be produced, because all the flank cutting blades are defined downwardly by the web of the profile. In the case of great height of the profiles that are to be produced, this will lead to wide spacings of teeth, which has the consequence that, with a given number of broach cutting teeth, the broach becomes too long. If however the spacing of teeth is reduced in spite of too great a height of the profile that is to be produced, this may lead to grinding problems. Within certain dimensional ranges of the profiles, exploiting the advantages of the design of the generic type will therefore be restricted. In particular in the case of twist broaching, the use of these broaches requires broaching machines of high rigidity and accurate twist actuation.
For elimination of the described problems, EP 1 160 040 A1 teaches, in an internal broach of a similar type, to provide broach cutting teeth with flank cutting blades that adjoin the bottom cutting blades. The height of the flank cutting blades is considerably less than the depth of the total profile that is to be broached and substantially exceeds the pitch of the individual broach cutting tooth. These partial flank cutting blades are provided with flank relief surfaces. The sides, adjoining the partial flank cutting blades, of the broach cutting teeth are relieved surfaces. This is intended to help accomplish high flank curve accuracy, good self-guidance behavior being expected. However, the problem of broach deviation i.e., the problem of the central longitudinal axis of the broach deviating from its desired position relative to the work piece, cannot be solved in a satisfying fashion.